This invention relates to nucleating agents to aid crystallization of polyesters and to articles made of cellularized compositions of the crystalline polyesters.
Polyesters, especially polyethylene terephthalate (PET), are widely used for articles in which properties of non-toxicity, good heat stability, resistance to deformation and ease of manufacture are important. PET is very useful for dual ovenable (i.e., usable in conventional ovens as well as microwave ovens) food packages and trays. Incorporating gas into the polyester to provide a cellular structure also has advantages in many applications, and especially dual ovenable food trays. The cellular structure not only reduces the weight of polyester needed in sheet and trays, but also provides heat insulation. Food trays and other articles are generally made from polyester by thermoforming. In one process of thermoforming, a sheet of polyester is preheated to its deformation temperature and made to conform to the contours of a mold by vacuum assist (vacuuming it into the mold), air pressure assist (blowing it into the mold), plug assist and matched mold assist (forcing it into the mold). It is essential that the polyester be partially crystalline to maintain good dimensional stability, stiffness and other desirable properties. Crystallinity of about 15 to 32 percent after forming is customary. It is a happy coincidence that polyesters crystallize upon heating, so that the thermoforming step plus a period of heat treatment (holding the article at the molding temperature for a time) results in the desired crystallinity. Of course, the polyester must be properly selected, and have added to it a suitable crystallization nucleation agent, i.e. an agent (usually particles) which initiates and provides a starting point on which crystallites can form. Moreover, it is necessary to provide some improvement in impact strength to prevent or at least reduce shattering. This is usually achieved by adding a small amount of a polyolefin component to the polyester blend.
Gas addition to obtain a cellular structure is conveniently accomplished in an extruder used to form the sheet. While other methods can be used, the injection of gas (nitrogen is preferred for cost, availability, convenience and environmental friendliness) into the barrel of the extruder is customary. However, the composition and conditions must be carefully selected and controlled to get the desired result. Extruded sheet is normally fairly thin, for example about 0.030 inch. It is necessary, therefore, to have many very small bubbles; large bubbles result in a rough surface and, if too large, in holes in the sheet. Extrusion speed is also important. Extrusion cannot be faster than bubble formation. Since bubbles form around seed particles, a suitable nucleating agent of sufficiently small particle size can speed bubble formation, extrusion and thermoforming.
While adequate crystallinity (i.e. about 25 percent) can be obtained by thermoforming, especially with heat treatment in the mold, neither satisfactory impact strength nor economical cycle times can be achieved. Nor does extended heat treatment (i.e. 30 to 60 seconds depending upon temperature) give adequate crystallinity for easy release of a part from the heated mold. These are not new problems, and the industry has come a long way in overcoming the limitations. It is known to add polyolefins and inorganic nucleating agents, or to add only polyolefins, such as polyethylene or polypropylene, to speed crystallization and improve impact strength. There is, however, a never ending quest for yet better processing and product properties. Economics dictate ever faster cycle times. A nucleating agent that allows fast bubble formation and fast cycle time is, therefore, very desirable. We have discovered a nucleation/bubble formation system that accomplishes that.